Philosophers of science are becoming more sensitive to the claims about truth and rationality being made by sociologists of science. There is a tendency among some of these philosophers to dismiss such claims as irrelevant to philosophy of science and as self-refuting. Larry Laudan, in his 'arationality assumption', has captured the essence of positions which argue that sociology of science can only be concerned with scientific claims which are not rational (or, in some versions, 'not true'). I show that the arguments he gives in support of the arationality assumption are not sound, that the arationality assumption has absurd consequences, and I argue that the arationality assumption undermines the possibility of fruitful cooperation between philosophers and sociologists of science. I conclude that the arationality assumption should be rejected